Slurries result from the mixture of a contaminated fluid with a photoreactive catalyst. Irradiation of a slurry, with light of sufficient energy, creates the formation of electrons and holes on the surface of the photoreactive catalyst. Electrochemical modifications to the contaminated fluid result from such formations. Such electrochemical modifications are generally referred to as a photocatalytic reaction. Photocatalytic reactions are employed for numerous purposes, such as decomposition, photosynthesis, the oxidation of contaminants, the reduction of contaminants, the sterilization of bacteria, deposition of metals, and the like. For example, a photocatalytic reaction can serve to oxidize toxic organics into carbon dioxide and water.
A catalytic action results when a catalyst reduces the "activation energy" that is required to complete a chemical reaction. In photocatalytic reactions, activation energy is provided by the photon energy of incident band-gap light. Incident band-gap light is provided by visible and ultraviolet light. When incident band-gap light is absorbed by a photoreactive catalyst, electron and hole charge carriers pairs are produced within the photocatalytic particles. These charge carriers then perform reduction/oxidation ("redox") reactions with the chemical species. Thus, optically excited photoreactive catalysts, such as anatase TiO.sub.2, can drive a chemical reaction at substantially lower temperatures than would otherwise be required.
The prior art provides for the irradiation of large amounts of slurry at one time. By irradiating the slurry in such a manner, only photoreactive catalysts that are exposed to the light source--those particles which come into close proximity with the light source--are irradiated. Consequently, only some portions of a slurry can be subjected to a photocatalytic reaction at one time. For example, U.S. Pat. No. 5,174,877, issued to Cooper et al., discloses subjecting an entire slurry to a photocatalytic reaction at one time in a tank reactor. The slurry at the bottom of the tank reactor is continually shifted to the top of the tank reactor until all of the slurry is subjected to a photocatalytic reaction.
Moreover, the prior art has gone to great lengths to continually mix a slurry so that the catalyst is suspended and uniformly dispersed throughout the slurry. For example, U.S. Pat. No. 5,174,877, issued to Cooper, et al., provides for stirring impellers composed of various materials and geometries and disposed on the bottom of a reactor tank for maintaining catalytic particles in a suspended state within the slurry. This has proven to be a time consuming and inefficient undertaking.
Once a photocatalytic reaction has taken place, and contaminants destroyed from the contaminated fluid, it is necessary to segregate the photoreactive catalyst from the decontaminated effluent. The prior art that the applicant is aware of provides for filtration techniques through the utilization of a membrane composed of a polymeric materials, such as polypropylene, as disclosed in U.S. Pat. No. 5,118,422, issued to Cooper, et al.
Several serious problems are encountered when utilizing polymeric membranes in order to segregate photoreactive catalysts from a decontaminated effluent. Membranes composed of a polymeric material are unable to withstand elevated temperatures, as well as the application of elevated pressures. Inevitably, photoreactive catalysts collect in the membrane. Conventional methods have attempted to remove the build-up of photoreactive catalyst in the polymeric membranes by "back flushing" methods in order to minimize the forces that are exerted on a membrane. Due to the elastic nature of polymeric membranes, some catalysts even become embedded in the polymeric membranes.
Conventional back flushing methods require a significant volume of already recovered decontaminated effluent to be passed back through a polymeric membrane, over a substantial period of time, in order to remove photoreactive catalysts collections in the polymer. By requiring a substantial period of time to perform back flushing, the degree and volume of fluid that can be subjected to a photocatalytic reaction is significantly decreased. That is, a continuous flow process cannot be achieved. Further, polymeric membranes suffer from a high failure rate due to the wearing and stretching of the elastic polymer. Still further, polymeric membranes can be dissolved by various organics, and are unable to be sterilized without the use of chemicals. Moreover, polymeric membranes can be regularly used for only a few years, sometimes even months, before replacement is needed.
Even more troublesome is the effect of polymeric membranes on the system wherein they are employed. Conventional systems must include equipment, such as accumulators, buffer tanks, and centrifugal pumps, in order to allow for the back flushing of polymeric membranes. Mixing devices are also necessary to prevent stagnant catalytic particles from settling when back flushing operation are occurring.
Since back flushing requires a substantial expenditure of time, it is only undertaken on an infrequent basis. As a result, significant amounts of catalytic particles settle in the membranes. In turn, the time required for separating the catalyst from the decontaminated effluent is increased. Further, the concentrations of catalyst within a slurry significantly fluctuates.
In sum, conventional methods and systems do not provide for the efficient decontamination of fluids. Rather, conventional methods and systems are time consuming and inefficient. This results from an inability to subject catalysts, that are dispersed in a slurry, to irradiation in an efficient manner. Furthermore, conventional methods and systems are not able to continuously segregate decontaminated effluents from catalysts. This inability is magnified when high volumes of a slurry is provided for segregation, as is the case in large scale commercial applications.
It is thus highly desirable to provide for a method and system which overcomes the aforementioned problems of the prior art in order to enable the continuous, as well as efficient and effective, purification of contaminated fluids by a photocatalytic reaction.